General Information on Preventative Maintenance
To obtain satisfactory operation it is essential that the correct care is given to carbon brushes, its accessory parts and the conductors that supply current to electric motors.
- Depending upon the voltage, current being conducted and the size of the brush, the clearance between the end of the brush box and the commutator should be between 1,5mm to 2,0mm.
- It is essential that no particles become lodged between the commutator and the bearing surface of the brush.
- Brushes must be correctly aligned so that on the same brush arm they lie in the same neutral axis circumferentially so that wear is even over the commutator (staggering of brush track between positive and negative).
- Brushes can be damaged when a motor works under abnormal conditions i.e. when the commutator surface is rough, contaminated and/or distorted. These abnormal conditions can cause excessive sparking resulting in overheating of the brushes and commutator, followed by poor commutation; variation of loads, speed and develop vibration.
All the above problem conditions can be caused by either incorrect brush tension and/or incorrect brush grade. - If the brush box is not located in the correct neutral axis or if there is an electrical fault on the motor, the brush will overheat and cause burning on the edge of the brush with consequent damage to the commutator.
- Vibration or chattering of the brush can be caused by incorrect brush tension, distorted surface of commutator or slip ring, raised mica insulation and high risers.
- The carbon film build up can be caused by motors running under light or no load conditions. It is preferable that the current density in a brush is greater that 8 A/cm2. If for any reason a motor generator has to run for a lengthy period on light load, it may be necessary to de-brush or change the brush grade. Either way, please consult our brush experts before taken any of these measures.
Overloads are more difficult to cater for due to overheating, sparking and possible vibration of the brushes. - The spring tension on brushes is of vital importance to the correct functioning of a carbon brush. It is a common mistake to reduce the brush spring tension in an endeavour to reduce the streaking of the commutator. This tends to cause sparking due to the brush losing temporary contact with the commutator. Excessive brush spring tension increases the grooving of the commutator. Recommended spring tension for industrial applications varies from 130 to 300gm/cm2. On tractions applications the spring tensions range from 350gm to 2000gm.cm2.
The correct spring tension is of the most importance for the patina to form on the surface of the commutator or slip ring, this will create the ideal film for carbon brushes to slide on and commutate with minimum wear. - While spring tension of a brush within a brush box is of vital importance, so too is the clearance between brush and brush box. Free movement of the brush must be available at all times. Dust and oil within the brush box or on the brush surface can lead to the sticking of the brush in the brush box or prevent contact with the brush and the commutator.
- Commutator wear is usually associated with the hardness of the brush grade, however, this is not quite true as some of the most abrasive brush grades have a low Shore Hardness index. The manufacturer controls the abrasive properties of a brush and should be approached with regard to the cleaning properties of brushes. The most discernable difficulties with brushes are:
- Sticking brushes
- Loss of spring tension
- Dirty or corrosive contact surfaces.
The operating speed determines the design of the brush holders and associated parts. When motor fail to start, overheat, or develop low torques, it is necessary to check the brush condition. As a general rule, brush holders are set in the factory and should not need adjustment. However, when these wear, the complete unit should be replaced. Lifting arms of brushes on large machines should be checked periodically for possible wear.
